The mobile phone industry is moving rapidly toward providing hardware upgradability for mobile device components. One likely upgrade may involve adding a second Wi-Fi® radio, or more, to improve wireless local area network (WLAN) connectivity and provide better performance. While adding a second, or even multiple, WLAN integrated circuits (ICs) to create parallel data paths for the mobile device may improve throughput, challenges exist in that independent radios must contend for the same medium at the Media Access Control (MAC) Layer and at the physical (PHY) layer.
An example known multiple radio system 100 is shown in FIG. 1. The multiple radio system 100 includes a processor 101 with a first network interface 102 that uses a first MAC address and a second network interface 105 that uses a second MAC address. The network interfaces are software access points to the processor 101. A first radio system, WLAN subsystem 1 104, includes WLAN hardware, a WLAN subsystem 1 driver and interfaces with an antenna system. A second radio system, WLAN subsystem 2 107, also includes WLAN hardware, a WLAN subsystem 2 driver and may interface with the same antenna system as first radio system, or may have a separate antenna system. In either case, the WLAN subsystem 1 104 MAC Layer sends data transmissions 103 and also receives data via the network interface 102. The second radio system, WLAN subsystem 2 107 MAC layer similarly sends data transmissions 106 and also receives data via the network interface 105. The data transmissions 103 and data transmissions 106 are in contention at the MAC Layer and at the PHY layer in the wireless medium access, based on the backoff algorithm.
A timing diagram 200 in FIG. 2 provides an example of a parallel WLAN data stream for two radio transmitters (Tx 1 and Tx 2) that employs a Request-to-Send/Clear-to-Send (RTS/CTS) mechanism which is used to prevent the hidden node problem. The operations implemented in timing diagram 200 comply with IEEE 802.11 requirements. As shown, when the first radio transmitter (Tx 1) is ready to send data, an RTS packet is sent to the WLAN Access Point (AP). The transmitter Tx 1 must wait to receive the CTS before it begins to send data. After the data transmission is completed, the AP sends an acknowledgement (ACK) to Tx 1. The second transmitter Tx 2 must wait until the first transmitter receives it ACK (the “backoff” period), the DCF Interframe Space (DIFS) and a random backoff time before it sends its own RTS. Further delay time is added by the Short Interframe Space (SIFS) until the CTS is received and a second SIFS prior to the second transmitter Tx 2 being able to transmit data. As can be seen from the illustration of FIG. 2, Tx 2 must wait for Tx 1 to complete its transmission before Tx 2 can request a Network Allocation Vector (NAV Request) via an RTS. FIG. 3 illustrates an example scenario in which RTS/CTS is not used under the assumption that the hidden node problem is not an issue. The operations implemented in timing diagram 300 also comply with IEEE 802.11 requirements. As can be seen in FIG. 3, the second transmitter Tx 2 must wait for a minimum of the SIFS, a DIFS, and a Random Backoff duration before Tx 2 has an opportunity to transmit data. Therefore the second transmitter faces quite a long delay before it is able to engage in sending any data.